Not applicable
This invention relates to dot matrix printers, and more particularly to dot matrix printers including stationary arrays of printing heads for printing at high-speed and high-resolution.
Dot matrix printers typically include at least one print head with a plurality of individual printing elements arranged within the print head. A dot matrix printer typically actuates individual printing elements in the print head in a pattern of operation that is controlled by a stream of data in successive steps as the print head traverses a printing surface of a printing medium such as paper. During each step, the print head prints an area of dots and then move horizontally to a new position to print a succeeding area of dots. This process is repeated to produce a horizontal line of characters or other such image across the printing medium. After one horizontal line is printed, the print medium is typically incrementally moved in the vertical direction to permit another horizontal line of the image, such as a row of characters.
Therefore, dot matrix printers require successive actuation of one or more print heads typically including multiple printing elements arranged across a relative path of movement between the printing medium and the print head. One technique to progressively increase printing speed employs printing while moving in opposite directions back and forth in a rectangular path. Another technique employs multiple printing heads arranged side-by-side along a rectangular path. Another technique for increasing speed, employs double or multiple height print heads arranged across the rectangular path to simultaneously print two or more rows of characters during each traverse of the printing medium.
There are many examples of previous rearrangements of dot matrix print heads or their printing elements for increasing printing speeds and/or image resolution. For example, U.S. Pat. No.4,462,706 for STACKABLE DOT MATRIX PRINTING CARTRIDGE MODULES describes a stacked array of print heads that are stacked horizontally or vertically; U.S. Pat. No. 4,552,064 for DOT MATRIX PRINTERS AND PRINT HEADS THEREFOR describes a print head having the dimensions of a 34 pin head being 2.0 inches wide, 1.5 inches thick, and 14.2 inches in length; and U.S. Pat. No. 4,236,836 for DOT IMPACT PRINTER AND ACTUATOR THEREFOR describes a dot matrix printer in which 44 to 132 print heads are employed to print one line at a time.
U.S. Pat. No. 5,793,392 for PRINTING APPARATUS AND METHOD, which is assigned to the assignee of this application, describes a printing system for printing an image having a width on a printing medium. The printing system includes a print head array having multiple columns of print heads. Each column includes a plurality of print heads having varying positions in a first dimension in the print head array for printing in a corresponding printable column area of the printing medium and having a corresponding printable column width. The multiple columns of print heads are arranged for printing throughout the image width. A first mechanism moves the printing medium relative to the print head array in the first dimension to cause selected non-contiguous portions of a printable segment along a second dimension substantially perpendicular to the first dimension to be printed in each printable column area by the print heads. Further movement in the first dimension causes selected non-contiguous portions of multiple defined printable segments to be printed to fill the corresponding image portions of each column area. A second mechanism moves the print head array relative to the printing medium in the second dimension. A movement in the second dimension not more than the widest distance between any two non-contiguous portions of any printable segment in combination with the movement in the first dimension is sufficient to print all printable segments contained in the image.
Just as many types of dot matrix printers are available, a corresponding variety of print head types exist. For example, in electro-mechanical actuator impact print heads, a plurality of print wires are selectively driven by corresponding solenoids to impact a printing surface directly with or through a transfer ribbon. A commercially popular type of print head is an ink-jet print head which uses a number of individual ink-jets to pulse droplets of ink in spatial combinations to print characters as a sequence of dots. Another type of dot matrix print head is the thermal printer in which printing is carried out by contact of multiple heated printing elements to heat sensitive paper or to an intervening thermal transfer ribbon to print data on ordinary paper.
The ink-jet print head is typically mounted on a carriage that moves substantially perpendicular to a media motion direction, to enable an ink-jet type dot matrix printer to produce a line of characters or type. An advantage of the ink-jet print head is that other than the movement of the carriage and the drops of ink moving through the ink-jet print head, there are no moving parts such as in the electro-mechanical actuator impact print head. Another advantages of the ink-jet print head are its relatively high image quality, color purity, and low cost. Unfortunately, ink-jet printers print relatively slowly.
To increase printing speed, U.S. Pat. No. 5,907,338 describes a media-width ink jet print head having four rows of nozzles for ejecting four colors of ink. Unfortunately this print head is very expensive and still requires at least two printing passes to produce a high-resolution image.
As described above, skilled workers have approached the problem of increasing the printing speed of ink-jet printers in various ways including developing faster print heads, increasing the number of print heads per printer. These approaches have achieved printing speeds several orders of magnitude greater than those achievable twenty years ago. There are, however, significant design and manufacturing problems associated with further increasing the throughput of individual print heads or the number of print heads per printer.
Increasing the number of ink-jet nozzles per print head as well as increasing the frequency at which each nozzle is able to place dots on the print page increases the printing speed of individual ink-jet print heads. Currently, individual high-resolution print heads have ink-jet nozzle arrays for one, three, four, and six colors. It has been either infeasible or prohibitively expensive to manufacture high-density ink-jet nozzle arrays wider than one inch.
Increasing the number of print heads in a printer also increases the printing speed of a printer. Many color ink-jet printers employ a linear array of four or six single-color print heads or a linear array comprising a single-color and a multicolor print head. Some color ink-jet printers, such as ones described in the afore-mentioned U.S. Pat. No. 5,793,392, employ a linear or a two-dimensional array of twelve single-color print heads.
There are several significant problems encountered when trying to increase the number of print heads in a printer. A first problem is dot placement precision, and a second problem is the cost of manufacturing a precisely aligned array of print heads.
As a print head array scans bilaterally across the width of the page, significant error accumulates in the accuracy of placing the dots of the printed image. This dot placement error also accumulates due to reversing the direction of the print head array and the incremental movement of the print medium through the printer.
Suppose that the columns of print heads in the print head array evenly partition the entire width of the printable image area and the rows of print heads evenly partition a small segment of the length of the printable image area. Then, the dot placement error Ei is represented mathematically as follows:
Let w and l be the width and length of the printable image area, respectively. Let hw and hl be the width and length of the printable image area that an individual print head in the array prints, respectively. Let R be the number of rows of print heads in the array. Let e be the dot placement error accumulated across hw due to a single unilateral pass of the print head array relative to the print medium. Let r be the dot placement error accumulated across hl due to reversing the direction of the print head array while bilaterally scanning. The errors e and r accumulate for every pass of the print head array, except the last pass for r.
Let p be the number of unilateral passes of the print head array across hl. Then, the maximum dot placement error accumulated due to the bilateral scanning of the print head array is:             l      ⁢              xe2x80x83            ⁢              p        ⁡                  (                      e            +            r                    )                            Rh      l        -  r
The incremental movement of the print medium through the printer is the cause of substantially more dot placement error. Let i be the error accumulated across hl due to the incremental movement of the print medium through the printer. Let j be the dot placement error accumulated across Rxe2x88x921 rows of print heads due to the incremental movement of the print medium through the printer in order to print the next segment of the printable image area. Then, the maximum dot placement error accumulated due to the incremental movement of the print medium across l is:             l      ⁢              xe2x80x83            ⁢              (                  i          +          j                )                    Rh      l        -  j
Therefore, due to the Pythagorean theorem, the maximum possible dot placement error E1 due to bilateral scanning, reversing directions and incremental print medium movement is:       E    1    2    =                    (                                            l              ⁢                              xe2x80x83                            ⁢                              p                ⁡                                  (                                      e                    +                    r                                    )                                                                    R              ⁢                              xe2x80x83                            ⁢                              h                l                                              -          r                )            2        +                  (                                            l              ⁢                              xe2x80x83                            ⁢                              (                                  i                  +                  j                                )                                                    Rh              l                                -          j                )            2      
The dot placement error is not so noticeable when there are but a few rows of print heads in the print head array. The dot placement error for dots placed by adjacent rows of print heads, relative to one another, is small even though the absolute dot placement error across the entire printed image is large.
However, the problem is quite different when adding more and more rows of print heads to the print head array. After the two-dimensional print head array prints a segment of the printable image area Rhl in length, the entire array must move a distance of (Rxe2x88x921)hl to continue printing the remainder of the image. The accumulated dot placement error at the bottom of each segment of the printable image area accompanied by the incremental movement error j creates unacceptable artifacts, distortions and banding across the width of the image between segments of the printable image area.
What is still needed, therefore, is a low-cost printing system that rapidly provides high-quality, high-resolution color images.
An object of this invention is, therefore, to provide a low-cost printing apparatus and method employing a stationary print head array that rapidly provides high-quality, high-resolution color images.
Another object of this invention is to provide a stationary print head array manufacturing method.
A further object of this invention is to provide a software device driver method for coupling printers to various print head array configurations.
Stationary print heads of this invention employ an array of conventional print heads, which are designed to bidirectionally scan horizontally while a print medium incrementally moves vertically. The print heads are reoriented 90 degrees relative to the conventional orientation such that the nozzle arrays in each print head are aligned perpendicular to the direction of print media movement. The array of reoriented print heads are positioned in at least a first row and a second row with the print heads in the first row spaced apart across the width of a printable image area. The print heads in the second row are similarly spaced apart but offset from the print heads in the first row. As the print medium continuously moves in one direction through the printer, the first and second rows of print heads print a wide swath comprising an entire linear row of dots across the width of the print medium without any gaps in the printed image. Continuously moving the print media, without conventional incremental movements or bidirectional print head scanning provides significantly increased printing speed and improved dot placement accuracy.
The manufacturing method of this invention starts with an imprecisely aligned array of print heads, precisely measures their nozzle jetting positions within the array, and compensates for their imprecise alignment with a software device driver. No matter how large the print head array, the precision afforded by this manufacturing method is better than the manufacturing tolerances used to precisely align much smaller arrays employed for printing photographic quality images. Any gaps or overlaps in the dot placement caused by imprecise alignment of the print heads in the array is compensated for by having some print heads partially overlap the coverage areas of adjacent print heads. The device driver then steers and times printing to the appropriate nozzle(s) in the appropriate print head(s) to eliminate the gaps or overlaps.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceeds with reference to the accompanying drawings.